When a real-life three-dimensional object is imaged onto two dimensions, the object undergoes perspective distortion. For example, perspective distortion occurs when photographing an object onto film or into a digital camera. This perspective distortion includes effects such as foreshortening, where identically sized objects become smaller the further they are from the camera and receding railroad tracks become narrower with increasing distance and appear to converge.
Another aspect of the distortion is referred to as keystoning. Keystoning can be seen in the progressive narrowing of the width of a skyscraper with height when it is photographed from sidewalk level. When the camera is not centered on the object the same keystone effect also occurs laterally. For example when the camera is pointed at an external vertical corner of a building, the adjacent sides decrease in size with distance from the corner. The perspective distortion of an object or image is governed by three orthogonally disposed vanishing points, as shown in FIG. 1. In the example of FIG. 1, the perspective distortion of object 100 is governed by three vanishing points 102, 104, and 106, each of which is orthogonal to the other two vanishing points.
Frequently, because of limitations on the physical location of the camera, perspective distortion cannot be avoided during image capture. When photographing architecture, for example, the arrangement of the surrounding buildings, streets and traffic, as well as the inability to access a vantage point of suitable height can constrain the position of the camera in a manner that causes significant perspective distortion. While it is possible to compensate for such difficulties with shift and tilt lenses, this specialized and expensive equipment is not usually possessed by a typical photographer. Moreover, these types of lenses often suffer from disadvantages of their own, such as vignetting, lack of autofocus and complexity of use.
Thus, it is common for images to be acquired with perspective distortion that the photographer seeks to correct or reduce by digital editing means.